Spectroscopy is the science of measuring radiation with a range of frequencies or wavelengths, and recording results which represent this range of frequencies or wavelengths as a spectrum. Spectroscopy can involve different ranges of radiation including microwaves, visible and invisible light, X-rays, or gamma rays, infrared radiation, ultraviolet radiation as well as other types of signals. In nuclear spectroscopy, the X-rays and gamma rays are frequently used to identify the presence of specific radioactive materials; this information is useful in determining both the type (natural, benign, threatening) and quantity (safe level, level of concern, health threat) of the material present.
Spectroscopy then, identifies a sample's relative degree of emission, transmission, absorption, or reflection over a range of radiation frequencies. For example, ordinary light is a radiation that is visible to the naked eye as it is reflected off of objects. Spectroscopy, however, often involves other ranges of radiation. Chemists often use mid-infrared radiation to determine the molecular content of a “sample” because different molecules absorb different amounts of the frequencies contained in such radiation. Each molecular species has a spectral “fingerprint” in the mid-infrared range. An instrument known as a spectrometer enables spectroscopic analysis. Earlier “wavelength dispersive” spectrometers rotated a dispersing element (grating or prism) through an arc so that all wavelengths within a desired range are presented to a detector. The industry subsequently developed spectrometers that use an interferometer to create a composite signal called an interferogram—a signal containing all frequencies in the entire spectrum—and then analyze the magnitude of each particular frequencies in that composite signal using the relatively complicated but well known mathematics of the Fourier Transform (FT). Such interferometer-based spectrometers are often called Fourier Transform Infrared spectrometers, or simply FTIR spectrometers.
Conventional gamma ray spectroscopy systems are comprised of a high voltage source that provides power to a means for detecting gamma rays emitted from a particular source of interest. A gamma ray emitted from a source of nuclear decay is converted by the detector into an electrical analog pulse signal connected thereto. The analog pulse signal is typically amplified in a preamplifier and subsequently shaped in a shaping amplifier and stretched in a pulse stretcher. After the analog pulse signal is amplified, shaped, and stretched, it is converted into a digital signal by an analog to digital converter. The analog to digital converter (ADC) outputs an n-bit digital signal (e.g., a 12-bit digital signal) representing the energy of the detected gamma ray, which is then counted in a binning scheme to produce a histogram, i.e. an energy spectrum of the incoming gamma rays.
A more recent development, enabled by the development of high-speed and high-performance analog-to-digital converters, is the digital spectrometer, which changes this order: the analog pulse signal is typically amplified in a preamplifier, and then immediately digitized by an ADC. Then, a signal processing step is performed which combines amplification, shaping, and stretching, which is performed in the digital domain by multiplying, summing, or applying other mathematical transforms to the digital signal produced by the ADC. The result is still a digital signal which is counted in a binning scheme to produce histogram.
The entire instrument enabling this analysis is known as the spectrometer. FIG. 1 shows a typical spectrometer (100) as applied to nuclear measurements. The device shown includes a detector (120), which may be a scintillator, a solid-state detector, or another comparable measurement device. The signal 20 from this detector typically goes through an amplifier (30) before being sent to a suitable analog-to-digital converter (40). In a digital nuclear spectrometer, very little processing is done before the analog-to-digital converter, because the important signal-processing is implemented in a digital signal processor (50). The data coming from the digital signal processor is then delivered to a processor (60) which stores the collected histogram (or “spectrum”) in memory (70). It may further implement analysis firmware (61) which converts the spectrum into a set of analysis results (65). (Firmware here is taken to mean software that is implemented on a special-purpose computing hardware, rather than on a conventional personal computer or server.) These results are then sent to an output device (110) which may be local (a display) or remote (a computer or database).
One of the critical factors in nuclear spectroscopy is resolution; better resolution, which corresponds to lower noise, improves the success of the later analysis. Some factors that determine resolution are outside the control of the spectrometer, such as intrinsic detector properties. However, the spectrometer can implement a low or high resolution in the spectrum it stores, which is usually described in channels. Nuclear spectrometers with as few as 256 channels and as many as 16384 channels are common; while a larger number of channels improves the accuracy of analysis, it also increases the cost of the system and often makes processing slower. As a result, systems are often designed to use the smallest number of channels required for the application.
Detectors used for detecting gamma rays in nuclear spectroscopy systems include: Geiger-Muller tubes, sodium iodide scintillation detectors, plastic scintillators, silicon (lithium) detectors, gas flow proportional counters, germanium (lithium) detectors and hyper-pure germanium detectors. Geiger-Muller tubes are very inexpensive but have essentially no energy resolution. That is, an analog pulse signal from a Geiger-Muller tube does not differentiate between incoming gamma rays according to energy. In contrast, hyper-pure germanium detectors have excellent resolution and are extremely linear in terms of energy over a wide variety of energies. However, hyper-pure germanium detectors can cost tens of thousands of dollars, require liquid nitrogen for cryogenic cooling, and are quite large.
Sodium iodide scintillation detectors, and many other scintillation-type detectors, have reasonable energy resolution, are rugged, do not require cryogenic cooling, are physically small and have a reasonably low cost. Sodium iodide detectors are therefore desirable for use in many applications in medicine, radiation surveying, waste monitoring, and education. Unfortunately, sodium iodide detectors suffer from a variety of problems. Some designs, such as the one set forth in U.S. Pat. No. 5,608,222 to Hardy et al, teaches a device that optimizes these channels to match the resolution of other components.
The '222 patent discloses and claims an analog to digital conversion technique for spectroscopy that enables the use of detectors in which resolution varies as a function of energy. The device further comprises an analog to digital converter with relatively poor differential linearity without sacrificing the ability to determine the locations and magnitude of peaks within a spectrum. A transfer function that characterizes the dependence of the resolution of the system is used to convert data before the data is displayed in a histogram. The transfer function can also characterize repeatable non-linearities in the system and may be used for gamma ray spectroscopy with sodium iodide detectors. The transform function can be implemented in a digital circuit, an analog circuit, or in a firmware or software transform table.
U.S. Pat. No. 7,498,964 to Beyerle discloses an approach to spectroscopy that facilitates the transfer of signals between both analog and digital spectrometers and their sub-components. Although the data stream in this approach is converted between different forms throughout the disclosed system, there is only a single data stream, representing and resulting in a single histogram.
U.S. Pat. No. 7,247,855 to Castellane et. al. discloses a portable nuclear material detector generally includes a scintillating fiber radiation sensor, a light detector, a conditioning circuit, a frequency shift keying (FSK) circuit, a fast Fourier transform (FFT) circuit, an electronic controller, an amplitude spectral addition circuit, and an output device. A high voltage direct current (HVDC) source is provided to excite the light detector, while a separate power supply may be provided to power the remaining components. Portability is facilitated by locating the components of the detector within a handheld-sized housing. When bombarded by gamma particles, the radiation sensor emits light, which is detected by the light detector and converted into electrical signals. These electrical signals are then conditioned and converted to spectral lines. The frequency of a give spectral line is associated with a particular radioactive isotope, while the cumulative amplitude of all spectral lines having a common frequency is indicative of the strength and location of the isotope. All or part of this information (identity, strength, direction, and distance) may be provided on the output device.
U.S. Pat. No. 6,327,549 to Bingham et al. discloses and claims a differential correction apparatus for use with a spectroscopy device. The spectroscopy device simultaneously produces two histograms corresponding to the spectrum acquired. The first histogram contains the counts recorded by a differential correction method (DCM), giving the best estimate of the counts per channel in the absence of dead time. The second histogram is the error spectrum, giving the variance of the counts in each channel of the first spectrum. The two spectra have the same size, true acquisition time, and energy calibration with the only difference being the number of counts in each channel. By obtaining both histograms, it is asserted to be possible to both obtain an accurate spectrum when the energy peaks have varying decay times and retain the necessary information about the spectrum to allow the statistical error to be calculated. However, in some applications, good resolution is critical for one part of the incoming spectral signal, but not needed for another portion of the signal. In these circumstances, a modified design which provides both types of data would provide the best of both worlds.
U.S. Pat. No. 6,636,619 to Auth et al. teaches a spectrometer having an interferometer, a detector that produces a detector signal, and a dual-digitizer system including two analog-to-digital converters that simultaneously digitize low-gain and high-gain versions of the detector signal, and suitable data structures and associated firmware for merging the two resulting sets of digitized data into a single, high dynamic range set of data. The analog detector signal has a high gain circuit to produce a high gain analog signal. This is then digitized to a low gain analog signal using a first ADC converter to produce a set of low gain samples. The high gain analog signal is processed by a second ADC converter to produce a set of high gain samples, some of the high gain samples being below a pre-determined threshold and some of the high gain samples being above the predetermined threshold. The in-range high gain samples are merged with the low gain samples to produce a combined set of samples by identifying a plurality of pairs of low-gain and high-gain samples where the high-gain samples are below the predetermined threshold; normalizing the high-gain samples relative to the low-gain samples; and assembling a combined set of low-gain sample and high-gain samples. However, this implementation relies on two sets of analog signal processing hardware and ADC components, which makes it complex, expensive, and inflexible. A modified design which implements differences in processing digitally would address these limitations.